The present invention relates to a high-Cr containing ferrite heat resistant steel. In further detail, the invention according to the present application relates to a high-Cr ferrite heat resistant steel having not only an excellent long-term creep strength at a high temperature exceeding 650xc2x0 C., but also an improved oxidation resistance.
Conventionally, the creep strength of a ferrite based, heat resistant steel has been improved heretofore by converting the ferritic texture into a tempered martensitic texture having a higher creep strength.
However, a tempered martensitic texture is unstable at high temperatures because it undergoes textural change and becomes heterogeneous. This decreases the creep strength. Furthermore, dislocations present in the martensite accelerates the long term creep deformation. Thus, the texture is changed influenced by the heat applied at welding as to impair the creep strength at the welded portion
Although Cr (chromium) is known as an element effective for improving the oxidation resistance of a steel, the incorporation of Cr at a higher concentration of 12% by weight or more results in the generation of a xcex4-ferrite phase which decreases the creep strength and the toughness. Accordingly, austenite stabilizing agents such as Ni, Cu and Co, have been added to the ferritic heat resistant steel known heretofore in order to suppress the generation of xcex4-ferrite phase.
However, the addition of Ni or Cu lowers the transformation temperatures of austenite and ferrite. To achieve long term stability of the high-temperature strength, it is advantageous to set the tempering temperature higher after the normalization; however, the addition of Ni or Cu results in a lower tempering temperature because it thus lowers the transformation temperature of austenite and ferrite. Accordingly, it is practically unfeasible to add Cr at a quantity exceeding a concentration of 12% by weight.
The invention according to the present application has been made in the light of the aforementioned circumstances, and an object thereof is to provide a high-Cr ferrite heat resistant steel having excellent long-term creep strength at a high temperature exceeding 650xc2x0 C., and yet having an improved oxidation resistance.
As described above, a conventional ferritic heat resistant steel based on the tempered martensitic texture suffers an abrupt drop in creep strength because it undergoes a heterogeneous textural change in the vicinity of the grain boundaries when subjected to higher temperatures over 650xc2x0 C. for a long duration of time because of the unstable texture.
Accordingly, the inventors of the present invension extensively studied a means for achieving textural stability at higher temperatures. As a result, it has been found that the ferritic heat resistant steel having a greatly improved long term creep strength at high temperatures can be obtained by realizing a texture based on a ferritic phase and precipitating therein an intermetallic compound of a Laves phase, a xcexc phase, a "sgr" phase, or a compound represented by Ni3X, where X is Al or Ti. The present invention has been accomplished based on these findings.
More specifically, in accordance with the first aspect of the invention of the present application, there is provided a heat resistant high-chromium containing ferrite steel containing 13% by weight or more of chromium and based on ferritic phase and containing precipitates of intermetallic compounds.
Furthermore, according to a second aspect of the invention of the present application, there is provided, a heat resistant high-chromium containing ferrite steel above, wherein the intermetallic compound is at least one type of precipitates selected from the group consisting of a Laves phase, a xcexc phase, a "sgr" phase, or a compound represented by Ni3X, where X is Al or Ti.